The Comparative Chemical Evolution of an Isolated Dwarf Galaxy: A VLT and Keck Spectroscopic Survey of WLM

Creators: Leaman, Ryan; Venn, Kim A.; Brooks, Alyson M.; Battaglia, Giuseppina; Cole, Andrew A.; Ibata, Rodrigo A.; Irwin, Mike J.; McConnachie, Alan W.; Mendel, J. Trevor; Starkenburg, Else; Tolstoy, Eline

Abstract

Building on our previous spectroscopic and photometric analysis of the isolated Local Group dwarf irregular (dIrr) galaxy WLM, we present a comparison of the metallicities of its red giant branch stars with respect to the well-studied Local Group dwarf spheroidal galaxies (dSphs) and Magellanic Clouds. We calculate a mean metallicity of [Fe/H] =–1.28 ± 0.02 and an intrinsic spread in metallicity of σ = 0.38 ± 0.04 dex, similar to the mean and spread observed in the massive dSph Fornax and the Small Magellanic Cloud. Thus, despite WLM's isolated environment, its global metallicity still follows expectations for mass and its global chemical evolution is similar to other nearby luminous dwarf galaxies (gas-rich or gas-poor). The data also show a radial gradient in [Fe/H] of d[Fe/H]/dr_c = –0.04 ± 0.04 dex r_c^(-1), which is flatter than that seen in the unbiased and spatially extended surveys of dSphs. Comparison of the spatial distribution of [Fe/H] in WLM, the Magellanic Clouds, and a sample of Local Group dSphs shows an apparent dichotomy in the sense that the dIrrs have statistically flatter radial [Fe/H] gradients than the low angular momentum dSphs. The correlation between angular momentum and radial metallicity gradient is further supported when considering the Local Group dEs. This chemodynamic relationship offers a new and useful constraint for environment-driven dwarf galaxy evolution models in the Local Group.

Additional Information

© 2013 American Astronomical Society. Received 2011 December 31; accepted 2013 February 1; published 2013 April 4. We are indebted to the referee, Dr. Ivo Saviane, for thorough readings and many useful comments and suggestions which greatly improved this manuscript. R.L. acknowledges support from NSERC Discovery Grants to Don VandenBerg and K.V., and acknowledges financial support to the DAGAL network from the People Programme (Marie Curie Actions) of the European Unions Seventh Framework Programme FP7/2007-2013/ under REA grant agreement number PITN-GA-2011-289313. The authors acknowledge the International Space Science Institute (ISSI) at Bern for their funding of the team "Defining the full life-cycle of dwarf galaxy evolution: the Local Universe as a template," as well as useful discussions with associated team members. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement number PIEF-GA-2010-274151. A.B. acknowledges support from the Sherman Fairchild Foundation and The Grainger Foundation. R.I. gratefully acknowledges support from the Agence Nationale de la Recherche though the grant POMMME (ANR 09-BLAN-0228). E.S. and E.T. gratefully acknowledge Netherlands Foundation for Scientific Research (NWO) for financial support. The authors thank M. Shetrone, A. Ferguson, R. Läsker, S. Ellison, K. Bekki, J. Peñnarrubia, and G. van de Ven for useful discussions, and R. Carrera, M. Parisi, L. Pompéia, E. Kirby, M. Geha, and A. Koch for making public their data. FORS2 observations were collected at the ESO, proposal 072.B-0497. Additional data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: VLT:Yepun (FORS2), Keck:II (DEIMOS)

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